The purpose of this research is to determine the effectiveness of the novel H1Nl influenza (inactivated/killed formulation) vaccine among both HIV-infected and HIV-uninfected persons. The administration of the H1Nl vaccination is not part of the study's procedures, but is being given as part of routine care.

The investigators study is being conducted to evaluate and compare the seroresponses of the novel H1N1 vaccination among HIV positive and negative persons receiving the novel H1N1 vaccination as part of routine clinical care, with secondary objectives examining the impact of prior seasonal vaccinations on subsequent seroresponse to the novel H1N1 vaccination, determining potential reactions (local or systemic) to this new vaccine among patients, and assessing for potential immunologic/virologic changes (in CD4/HIV RNA levels) after H1N1 vaccination among HIV patients. Finally, the investigators will collect data on influenza-like illnesses (ILI) and H1N1 events during the study follow-up period and influenza isolates causing ILI events will be genetically characterized.

Inclusion Criteria:

• 18-50 years of age
• Receiving the novel H1N1 vaccine (killed formulation) as part of routine clinical care
• A military beneficiary who expects to remain in the local area for the next 6 months

Exclusion Criteria:

• Healthcare worker who is involved in direct patient care
• Acute febrile illnesses within 30 days prior to H1N1 vaccination (e.g., pneumonia, influenza, ILI)
• Diabetes type 1 or type 2
• Systemic steroid or immunosuppressive medication use within 4 weeks of vaccination
• Active diagnoses of a cancer (non-melanoma skin cancer allowed).
• History of organ transplant
• Chronic active hepatitis B or C
• Active illicit drug use or alcohol abuse
• Blood transfusion within the last year
• Allergy to eggs
• Previous significant adverse reaction (e.g., anaphylaxis) to the seasonal influenza vaccination
• History of serious reactions to any prior vaccination (e.g., Guillain Barre Syndrome (GBS)).
• Received another vaccination in the last 4 weeks (receipt of seasonal influenza vaccination is allowed)
• Among females of childbearing potential, pregnant or within 6 weeks of being postpartum
• History of ILI which was confirmed as an H1N1 infection

Males and females of all races and all military beneficiaries including active duty enlisted and officers (from all branches of service), retirees, and their dependents presenting for H1N1 vaccination as part of routine clinical care will be eligible for participation. Both HIV-positive and negative persons can participate. Since most of our HIV patients are 18-50 years of age, and seroresponse varies by age, we will enroll persons between the ages of 18-50 years.